The present invention relates to a surface acoustic wave device having an inter-digital electrode on a single crystal substrate, and, in particular, such a device enabling miniaturization, band-widening and excellent selectivity, namely, superior temperature characteristics.
In recent years, various kinds of mobile communication terminal, devices inclusive of cellular telephones, have come into widespead use. It is highly desirable to reduce this kind of terminal equipment in size and weight for enhanced portability.
In order to reduce the size and weight of terminal devices, their electronic parts must be substantially reduced in size and weight. For this reason, surface acoustic wave devices enabling size and weight reduction, namely, surface acoustic wave filters, are often used for high- and intermediate-frequency parts of terminal devices. Such devices are formed with an inter-digital electrode for exciting, receiving, reflecting and propagating surface acoustic waves on the surface of a piezoelectric substrate thereof.
Among characteristics important to a piezoelectric substrate used for surface acoustic wave devices are surface wave velocity (SAW velocity), temperature coefficient of center frequency in the case of filters or of resonant frequency in the case of resonators (the temperature coefficient of frequency: TCF), and electromechanical coupling factor (k2). The characteristics of typical piezoelectric substrates currently known for surface acoustic wave devices are set forth below in Table 1. For details regarding these characteristics, reference should be made to Yasutaka SHIMIZU, xe2x80x9cPropagation characteristics of SAW materials and their current applicationxe2x80x9d, the Transactions of The Institute of Electronics, Information and Communication Engineers A, Vol. J76-A, No.2, pp. 129-137 (1993). Hereinafter, the piezoelectric substrates for surface acoustic wave devices are referred to using the designations in Table 1.
As can be seen from Table 1, 64LN and 36LT have an SAW velocity of 4000 m/s or higher, and as described later, 64LN and 36LT are suitable to construct filters for high-frequency parts of mobile communication terminal devices.
Various systems are practically employed all over the world for mobile communications devices, typically cellular telephones, and are all used at frequencies of the order of 1 GHz. Therefore, filters used for high-frequency parts of terminal devices have a center frequency of approximately 1 GHz. A surface acoustic wave filter has a center frequency substantially proportional to the SAW velocity of the piezoelectric substrate used and almost inversely proportional to the width of electrode fingers formed on the substrate. To enable such filters to be operated at high frequencies, therefore, it is preferable to utilize substrates having high SAW velocities, for instance, 64LN and 36LT.
Also, a wide passband width of 20 MHz or more is required for filters used as high-frequency parts. To achieve such wide passband, however, it is essential for the piezoelectric substrate to have a large electromechanical coupling factor k2. For these reasons, much use is made of 64LN and 36LT.
On the other hand, mobile communication terminal devices use an intermediate frequency in the 70 to 300 MHz band. When a filter having a center frequency in this frequency band is constructed using a surface acoustic wave device, if the aforementioned 64LN or 36 LT is used as the piezoelectric substrate, the widths of the electrode fingers formed on the substrate have to be much larger than those of the aforementioned filter used as a high-frequency part.
More specifically, the following equation (1) roughly applies to the relationship among the width d of an electrode finger of a surface acoustic wave transducer that forms a surface acoustic wave filter, the center frequency f0 of the surface acoustic wave filter, and the SAW velocity V of the piezoelectric substrate used.
f0=V/(4d)xe2x80x83xe2x80x83(1)
If a surface acoustic wave filter having a center frequency of 1 GHz is constructed on the assumption that the SAW velocity is 4000 m/s, the width of the electrode finger thereof is calculated from the equation (1) to be
d=4000(m/s)/(4xc3x971000(MHz))=1 xcexcm
On the other hand, when an intermediate-frequency filter having a center frequency of 100 MHz is constructed using this piezoelectric substrate having an SAW velocity of 4000 m/s, the width of the electrode finger required for this is given by
d=4000(m/s)/(4xc3x97100(MHz))=10 xcexcm
Thus, the required width of the electrode finger is ten times as large as that for the high-frequency part filter. A large width of the electrode finger means that the surface acoustic wave intermediate-frequency filter itself becomes large. Therefore, in order to make a surface acoustic wave intermediate-frequency filter small, it is necessary to use a piezoelectric substrate having a low SAW velocity V as can be appreciated from the equation (1).
BGO referred to in Table 1 is known as a piezoelectric substrate having a very low SAW velocity. However, since the temperature coefficient of frequency TCF of a BGO piezoelectric substrate is extremely large, the BGO piezoelectric substrate is not suitable for constructing an intermediate-frequency filter for extracting one channel signal alone. This is because a large TCF value means that the center frequency of the surface acoustic wave filter varies greatly with temperature. Thus, a large TCF is unsuitable for an intermediate-frequency filter because undesired signals may possibly be extracted from another channel adjacent to the desired channel.
As described above, one problem with the conventional surface acoustic wave device is that in the case where a piezoelectric substrate having a large electromechanical coupling factor such as 64LN and 36LT is used, it is possible to make the passband thereof wide but the device size becomes large since the substrate has a high SAW velocity. Another problem is that when the aforementioned BGO substrate having a low SAW velocity is used to achieve device size reduction, sufficient selectivity cannot be obtained because the absolute value of temperature coefficient of frequency TCF is too large. In either case, characteristics sufficient for any intermediate-frequency surface wave acoustic filter cannot be achieved.
On the other hand, ST quartz crystal referred to in Table 1 is known as a piezoelectric substrate having a relatively low SAW velocity. Although the effective SAW velocity of a piezoelectric substrate is influenced by the structure of the electrode finger formed on the substrate, it is known that the SAW velocity of ST quartz crystal is generally 3130 to 3155 m/s and since this value is approximately three-fourths the SAW velocity of 64LN or 36 LT, ST quartz crystal is suitable for miniaturization.
In view of these facts, most conventional intermediate-frequency surface acoustic wave devices for mobile communication terminal devices are constructed using an ST quartz crystal piezoelectric substrate.
However, as apparent from Table 1, the electromechanical coupling factor k2 of ST quartz crystal is 0.17%, particularly small among piezoelectric substrates. Small k2 means that only a filter having a narrow passband is achievable.
Nevertheless, main devices adopted so far for mobile communication, namely, cellular telephones, are analog systems with a very narrow channel width of, for instance, 12.5 kHz according to the Japanese NTT standard, 30 kHz according to the U.S. AMPS standard and 25 kHz according to the European TACS standard. Thus, the fact that the aforementioned ST quartz crystal has a small electromechanical coupling factor k2 has caused no problem whatsoever.
In recent years, however, digital mobile communication systems have been developed and put into practical use. These systems have won very rapid acceptance because of their ability to make effective use of frequency resources, compatibility with digital data communications, and so on. The channel width of this digital mobile communication system is very wide, for instance, 200 kHz in the European cellular telephone GSM and 1.7 MHz in the European cordless telephone DECT modes. It is difficult to construct an intermediate-frequency filter having the wide band required in the digital mobile communication system when an ST quartz crystal substrate having a small electromechanical coupling factor k2 is used for the surface acoustic wave filter.
Further, when an ST quartz crystal substrate is used for a surface acoustic wave filter, it is not only difficult to construct a wide-band intermediate-frequency filter because of the small electromechanical coupling factor thereof but also difficult to make the surface acoustic wave filter small since the SAW velocity thereof, though relatively small, is not small enough.
It is therefore an object of the present invention to provide a surface acoustic wave device enabling miniaturization, band-widening and excellent selectivity, namely, superior temperature characteristics.
The above and other objects of the present invention can be accomplished by a surface acoustic wave device comprising a substrate and an inter-digital electrode on a surface of the substrate, wherein the substrate is formed by cutting out of a single crystal represented by a chemical formula, Laxcex1Taxcex2Gaxcex3Oxcex4(2.9xe2x89xa6xcex1xe2x89xa63.1, 0.48xe2x89xa6xcex2xe2x89xa60.52, 5.4xe2x89xa6xcex3xe2x89xa65.6 and 13.8xe2x89xa6xcex4xcex3xe2x89xa614.2), a cut angle of the substrate cut out of the single crystal and a direction of propagation of surface acoustic waves on the substrate represented in terms of Euler""s angles (xcfx86, xcex8, "psgr"), xcfx86, xcex8 and "psgr" are found in one of a first area represented by xe2x88x925 degrees xe2x89xa6xcfx86xe2x89xa65 degrees, 135 degrees xe2x89xa6xcex8xe2x89xa6150 degrees and 20 degrees  less than "psgr" less than  25 degrees and a second area represented by xe2x88x925 degrees xe2x89xa6xcfx86xe2x89xa65 degrees, 135 degrees xe2x89xa6xcex8xe2x89xa6150 degrees and xe2x88x9225 degrees  less than "psgr" less than  xe2x88x9220 degrees, respectively, and a normalized thickness h/xcex obtained by normalizing a thickness h of the inter-digital electrode finger with a wavelength of the surface acoustic wave exceeds 0.5% and is equal to or smaller than 4%.
The present invention is based on the experimentally acquired knowledge that when a single crystal represented by the chemical formula, Laxcex1Taxcex2Gaxcex3Oxcex4(2.9xe2x89xa6xcex1xe2x89xa63.1, 0.48xe2x89xa6xcex2xe2x89xa60.52, 5.4xe2x89xa6xcex3xe2x89xa65.6 and 13.8xe2x89xa6xcex4xe2x89xa614.2) (hereinafter referred to as xe2x80x9can LTG single crystalxe2x80x9d) has a specifically combined cut angle and direction of propagation of surface acoustic waves such that when represented by xcfx86, xcex8, and "psgr" in terms of Euler""s angles (xcfx86, xcex8, "psgr"), they are present within the first area or the second area, the temperature dependence of the center frequency of a filter comprising the single crystal or that of the resonant frequency of a resonator filter comprising the single crystal can be approximated by a quadratic curve, and such properties as expressed in terms of an SAW velocity of less than 3000 m/s and an electromechanical coupling factor of greater than 0.4% can be obtained.
The present invention is further based on the experimentally acquired knowledge that when the LTG single crystal has a specifically combined cut angle and direction of propagation of surface acoustic waves such that when represented by xcfx86, xcex8, and "psgr" in terms of Euler""s angles (xcfx86, xcex8, "psgr"), they are present within the first area or the second area, the temperature corresponding to the peak of the quadratic curve, namely, the temperature at which the change of the center frequency or the resonant frequency is minimized, closely correlates with the normalized thickness h/xcex. of the inter-digital electrode finger (wherein h is the thickness of the inter-digital electrode finger and xcex is the wavelength of the surface acoustic wave at the center frequency or the resonant frequency) and that it is necessary to determine the normalized thickness h/xcex so as to exceed 0.5% and be equal to or less than 4% in order to set the temperature corresponding to the peak of the quadratic curve in the vicinity of room temperature and achieve a surface acoustic wave device which shows an excellent temperature stability at an ambient temperature at which it is normally used.
In the present invention, xcfx86 in the first area preferably ranges from 20.5 degrees to 23.5 degrees.
In the present invention, xcfx86 in the second area preferably ranges from xe2x88x9223.5 degrees to xe2x88x9220.5 degrees.
In the present invention, the normalized thickness h/xcex preferably ranges from 0.7% to 2%.
The LTG single crystal used in the present invention is generally represented by the chemical formula, Laxcex1Taxcex2Gaxcex3Oxcex4(2.9xe2x89xa6xcex1xe2x89xa63.1, 0.48xe2x89xa6xcex2xe2x89xa60.52, 5.4xe2x89xa6xcex3xe2x89xa65.6 and 13.8xe2x89xa6xcex3xcex414.2) and is known as a substituted cryst of a well-known langasite single crystal (La3Ga5SiO14) (Extended Abstracts (The 44th Spring Meeting, 1997); The Japan Society of Applied Physics and Related Societies).
The present invention is intended to achieve a surface acoustic wave device having excellent characteristics by employing an LTG single crystal as the substrate of the surface acoustic wave device, selecting the cut angle of the crystal and the direction of propagation of surface acoustic waves within a specific range and selecting the normalized thickness of the inter-digital electrode finger within a specific range.
In the present invention, if manufacturing errors are not considered, the LTG crystal most preferably has such a composition that xcex1 is equal to 3, xcex2 is equal to 0.5, xcex3 is equal to 5.5 and xcex4 is equal to 14, namely, La3Ta0.5Ga5.5O14. If manufacturing errors are considered, the composition ratios between the respective elements may be slightly varied within the range of 2.9xe2x89xa6xcex1xe2x89xa63.1, 0.48xe2x89xa6xcex2xe2x89xa60.52, 5.4xe2x89xa6xcex3xe2x89xa65.6 and 13.8xe2x89xa6xcex4xe2x89xa614.2 insofar as the technical advantages of the present invention are not deteriorated. Further, the LTG single crystal may contain unavoidable impurities such as Al, Zr, Fe, Ce, Nd, Pt and Ca. The method of manufacturing an LTG single crystal is not particularly limited and an ordinary crystal growing process such as the CZ process can be employed for manufacturing the LTG single crystal.
It is to be noted that the LTG single crystal is a trigonal system, and so mutually equivalent combinations of Euler""s angles exist due to crystal symmetry. In the trigonal system substrate, xcfx86=120 degrees to 240 degrees and xcfx86=240 degrees to 360 degrees (xe2x88x92120 degrees to 0 degree) are equivalent to xcfx86=0 degree to 120 degrees; xcex8=360 degrees to 180 degrees (0 degree to xe2x88x92180 degrees) is equivalent to xcex8=0 degree to 180 degrees; and "psgr"=270 degrees to 90 degrees is equivalent to "psgr"=xe2x88x9290 degrees to 90 degrees. For example, xcfx86=130 degrees and xcfx86=250 degrees are equivalent to xcfx86=10 degrees; xcex8=330 degrees is equivalent to xcex8=30 degrees; and "psgr"=240 degrees is equivalent to "psgr"=60 degrees.
Further, in the trigonal system substrate, the characteristics thereof at all cut angles and in all propagation directions can be learned by determining characteristics thereof in the range from 0 degree to 30 degrees.
Therefore, in order to learn the characteristics thereof at all cut angles and in all propagation directions, it is sufficient to study the characteristics thereof only in the range of xcfx860=0 degree to 30 degrees, xcex80=0 degree to 180 degrees and "psgr"0=xe2x88x9290 degrees to 90 degrees. Based on the combination (xcfx860, xcex80, "psgr"0) an equivalent combination exhibiting the same characteristics in the range of xcfx86=30 degree to 120 degrees can be learned. Concretely, in the range of 30 degrees xe2x89xa6xcfx86xe2x89xa660 degrees, (xcfx86, xcex8, "psgr") equivalent to (xcfx860, xcex80, "psgr"0) can be obtained by xcfx86=60 degrees xe2x88x92xcfx860, xcex8=180 degrees xe2x88x92xcex80 and "psgr"="psgr"0 in the range of 30 degrees xe2x89xa6xcfx86xe2x89xa660 degrees, (xcfx86, xcex8, "psgr") equivalent to xcfx860, xcex80, "psgr"0) can be obtained by xcfx86=60 degrees +xcfx860, xcex8=180 degrees xe2x88x92xcex80 and "psgr"=xe2x88x92"psgr"0; and in the range of 90 degrees xe2x89xa6xcfx86xe2x89xa6120 degrees, (xcfx86, xcex8, "psgr") equivalent to (xcfx860, xcex80, "psgr"0) can be obtained by xcfx86=120 degrees xe2x88x92xcfx860, xcex8=xcex80 and "psgr"=xe2x88x92"psgr"0. As a result, based on the aforementioned symmetrical property, the characteristics of the LTG single crystal substrate for all (xcfx86, xcex8, "psgr") can be learned.
Set out below are exemplary equivalent combinations.
Equivalent to (0 degree, 140 degrees, 25 degrees) are (60 degrees, 40 degrees, 25 degrees), (60 degrees, 40 degrees, xe2x88x9225 degrees) and (120 degrees, 140 degrees, xe2x88x9225 degrees) and since xcfx86=120 degrees is equivalent to xcfx86=0 degree, (0 degree, 140 degrees, xe2x88x9225 degrees) is also equivalent to (0 degree, 140 degrees, 25 degrees).
The first area and the second area include all combinations of equivalent (xcfx86, xcex8, "psgr") obtained in the above described manner.
It is to be noted that the wavelength xcex of a surface acoustic wave is determined by the frequency to which the surface acoustic wave device is applied and the speed of sound of the substrate. In the preferred frequency band to which the present invention is applied the wavelength xcex is generally 8 to 60 xcexcm.
In general, the surface acoustic wave device according to the present invention lends itself well to filters used in the frequency band of 10 to 300 MHz.
Further, the surface acoustic wave device according to the present invention is also useful for making a surface acoustic wave delay element small, because of the low SAW velocity thereof.